COMMUNICATIONS TO THE EDITOR
4428
J
t
1
I
1
4
a
* Fig. 1.-The unit cell of K ~ B ~ z Hwith ~ z , the center of symmetry a t the origin. The icosahedra have been drawn a t two-thirds of their proper size in order to avoid overlap. Icosahedra a t level zero are indicated by lighter lines, and those a t level '/z are indicated by heavier lines. Double circles indicate K + a t levels and 3/4.
by 24 H atoms at a distance of 2.29 8.( u = 0.06 A.). The B-H bond length of 1.07 b. (u = 0.06 k . ) is short, but just within 3 standarG deviations of the average B-H distance of 1.21 A. found in boron hydrides and their derivatives.6 We wish to thank Dr. 31. F. Hawthorne for supplying us with the sample, and the National Tnstitutes of Health and the Air Force Office of Scientific Research for financial support.
Vol. g 2
4 6 @Pm) H-. Fig. 1.-Proton magnetic resonance spectra at 60 Mc./sec. of 1,2-dithiane-~,j-I-Iz~ referred to benzene.
resonance. This molecule gives, however, a very complicated n.m.r. spectrum, and therefore we prepared a deuterium-substituted derivative (111). C---CH,--OH C-CH,--OH
C,-CHa--OD 11' C-CHn-OD
DaC-CH?--OD
HBr
DyC-CHa--OD DzC-CH,--SH
(?;H2)2C=S
11'
1
I
DPC-CH,-SH
Da0
---+
-
D,, PtO?
+
25", AcOEt
(1) HzO, KaOH
I?
EtOH-H,O
--f
( 2 ) HC1
D,C-CD, HZC(
)CIJ2 S
S (111)
From low temperature studies of a 1 ;IT solution in (G) W N Lipscomb, "Advances in Inorganic and Radiochemistry," Vol 1, Academic Press, New York, N . Y , p 117. carbon disulfide we obtained data which provide an estimate of the rotational barrier for this molecule.2 DEPARTMENT OF CHEMISTRY UNIVERSITY JEFFREY A . \VUXDERLICH At temperatures below -50' the spectrum conHARVARD CAMBRIDGE 38, MASS. \VILLIAM N.LIPSCOMB sists of a quartet of the AB type3 (Fig. 1). The RECEIVED J U L Y 5 , 1960 maximum separation of the two inner peaks is 10.4 c./sec., and the coupling constant is 13.0 c./sec. THE ENERGY BARRIER BETWEEN THE When the temperature is raised above --50°, the ENANTIOMERS OF 1.2-DITHIANE' peaks in the quartet start to converge, and a t - 43 Sir: the spectrum consists of one single broad peak. AS The dihedral angle in an organic disulfide can the temperature increases above - 43 ', the peak produce two different molecular conformations, I sharpens and remains constant up to the highest temperature studied (150 "). and 11. From the low temperature pattern the chemical {C,) shift between the axial and equatorial hydrogens, ya--ye, was calculated as 18.5 c./sec. We assumed that the following relation for the rate of intercon~ersion~ holds approximately a t - 43 ', the temperature where the peaks coalesce
,-. _y'
k = r(va -
I
I1
These are optical antipodes, and their stability depends on the height of the rotational barrier hindering free rotation about the disulfide bond. We have studied the interconversion between the enantiomers of 1,2-dithiane by nuclear magnetic (1) The work described in this paper was supported in part b y the U.S. Atomic Energy Commission.
ve)/di
From this we calculated the rate constant k sec.-l.
=
41.1
( 2 ) A Varian high-resolution n.m.r. spectrometer V-1311, operating a t GO hIc./sec. was used to obtain the spectra. (3) J . A. Pople, W. G. Schneider and H. J. Bernstein, "High Resolution Nuclear Magnetic Resonance," McCraw-Hill Book Company, N e w York, S.Y., 1969, p. 119. (4) H. S. Gutowsky and C. H. Holm, J . Chem. Phys., 26, 1228 (19SG).
Aug. 20, 1960
COMMUNICATIONS TO THE EDITOR
If the interchange from one isomer to the other
is treated as a typical rate process, and the transmission coefficient is equal to unity, then Eyring’s equation gives A F = 11.6 kcal./mole. Thus the molecular conformations have a very short lifetime. Even a t -43” the half-life for the enantiomers is only 0.02 sec. Rotatory dispersion in a disulfide absorption band indicating the asymmetry resulting from restricted rotation about the disulfide bond has indeed been observed in substances containing another asymmetric-inducing center.6 The same measurements on cyclohexane6 with similar assumptions give a value of 9.7 kcal./mole for AF*. If the six-membered rings are inverted by passing through intermediate boat forms, only one methylene opposition is involved in the transition through the maximum barrier in dithiane as compared to four in cyclohexane. Thus, one estimation of the AF* for disulfide rotation alone (in the cyclic compound) might be about 9.2 kcal./mole. Since the maximum dihedral angle in the six ring is likely to be about 60°, the energy difference ( A F * ) between 0 ” and 90’ (assuming a cos dependence’) would be 12.3 kcal./mole. The entropy of activation (AS*) for this reaction is, however, not known, and we must await a direct measurement of the temperature coefficient to evaluate the heat term.8 Since the entire transition in the spectrum from a single line to a quadruplet occurs between -40 and -50°, precise temperature control and simpler spectra would be desirable. Experiments toward this end are in progress.
4429
The 60-mc. spectrum shows that I has three types of protons in the ratio of 1:2 :2. Several structures can be eliminated immediately. The structure with the allyl group u-bonded to Pd should have four types of protons. The structure which has an additional T-Pd bond should have four or five different kinds of protons. Also excluded is the possibility of rapid allylic rearrangement observed3 for allylmagnesium bromide which has only two types of protons. The structure consistent with the observed n.m.r. spectrum is the non-classical one shown where the dotted lines are electron deficient bonds.
2
In the calculation of the AM2X2 system, all offdiagonal matrix elements between P m and P n were neglected when they differed in any of the total spin components F,(A), Fz(M) or F,(X) because the chemical shifts between protons were many times greater than the coupling constants. At 60 mc. the spectra parameters in C.P.S. are 6a = 52.5, 6~ = 135.2, 6x = 197.4
IJAMI= 6.4, ~ J A = x ~12.8and ~ J Msex 0~
The corresponding values obtained a t 15.1 mc. are 6~ = 13.0, 6~ = 3 3 . 8 6 ~= 51.0
11~~1 = 6.5, l J . ~ x /= 1 2 . 0 a n d ~ J M = x 0~
( 5 ) C. Djerassi, “Optical Rotatory Dispersion,” McGraw-Hill Book Company, Inc., New York, h7.Y., 1960, p. 223. (6) F. R . Jensen, D. S. iioyce, C. H. Sederholm and A. J. Berlin, THISJOURNAL, 82, 1256 (1960). (7) W. G. Dauben and K. S. Pitzer in “Steric Effects in Organic Chemistry,” ed. by M. S. Newman, John Wiley and Sons, Inc., New York, N. Y., 1956, p. 5 5 . (8) D. W. Scott, H. L. Finke, M. E. Gross, G. R. Guthrie and H. M. Huffman. THIS JOURNAL, 72, 2424 (1950); D. W. Scott, H. L. Finke, J. P. McCullough, M. E. Gross, R. E. Pennington and G. I. Waddington, ibid., 74, 2478 (1952); G. Bergson and L. Schotte,Arkiv. K e m i , 13,43 (1958). (9) Postdoctoral Fellow 1959-1960. O n leave of absence from the University of Uppsala, Uppsala, Sweden. Appointment supported by the International Cooperation Administration under the Visiting Research Scientists Program administered by the h‘ational Academy of Sciences of the United States of America.
From the 60-mc. spectrum of I1 it is clear that one of the X-protons has been replaced by the methyl group. In place of the 135.2-C.P.S. doublet in I, resulting from the two equivalent M-protons, is now a group of peaks centered a t 146 C.P.S. produced by protons M and MI. (The MI proton and the methyl group are bonded to the same carbon atom.) The total area is still equivalent to two protons. The nuclear system excluding the methyl group is considered to be AMM’X. The spectra parameters in C.P.S.are
STRUCTURES OF T H E ALLYL AND CROTYL PALLADIUM CHLORIDE COMPLEXES
values. A similar large difference of 40.2 C.P.S. between the a- and the @-protonsin the C5He ring was observed in (n-C6H5)( C ~ & ) C O . ~ H a , which has a higher chemical shift value than HP, is on the
6.4 = 57.9. 8%‘ = 146.1, 6~ = 143.1, Sx = 209.7 6CHa = 299, l J A Y ( = 6 . 3 , I J A 3 f ’ l = 12.0 DEPARTMENT OF CHEMISTRY AND GORAN CLAESON~ RADIATION LABORATORY LAWRENCE ~ J A= x ~11.5, ~ J M ’ = C6.0, H ~and / ~ J M=x 0~ UNIVERSITY OF CALIFORSIA GAYLORD M. ANDROES The protons M and X bonded to the same carbon BERKELEY 4, CALIFORNIA MELVINCALVIN RECEIVED JUNE 23, 1960 atom in I differ by 62.2 C.P.S.in their chemical shift
Sir: The allylpalladium chloride (I) and crotylpalladium chloride (11)complexes1are unusually stable for organometallic halides. Our nuclear magnetic resonance (n.m.r.) results favor a non-classical structure for each compound.2 Infrared spectra also support these assignments. (1) J. Smidt and W. Hafner, Angcw. Chem., 11, 284 (1959).
(2) A Varian Associates 60-mc. and 15.1-mc. high resolution n.m r . spectrometer and a Beckmann IR-7spectrometer were used in this work. All solutions were 10% in CDClr. The n.m.r. peaks were referred t o an external benzene standard. The complexes I and I1 were prepared as described previously1 and gave correct analyses. (3) J. E. Norlander and J . D. Roberts, THIS JOURNAL, 81, 1769 (1969). (4) M. L. H. Green, L. Pratt and G. Wilkinson, J . Chcm SOC.,3753 (1959).
